In a matrix for tissue repair comprising a biocompatible, bioerodable polymer, this invention provides the improvement wherein the polymer has a water solubility of about 0.01 to about 500 mg/mL at about 25xc2x0 C. and an adhesive strength of about 600 to about 150,000 Pa so that the matrix is tissue adherent. One such matrix comprises a polymer that also has a glass transition temperature of less than 0xc2x0 C. The improved matrices are useful for repairing tissues such as bone and cartilage, and for administering biologically active substances.
These improved matrices may further comprise a filler, a bioactive agent, or both.
In another aspect, this invention provides a pressure sensitive adhesive for tissue repair comprising (a) a biocompatible, bioerodable polymer which exhibits adhesive strength of about 600 to about 150,000 Pa, (b) a filler and (c) a bioactive agent. Further, this invention provides a pressure sensitive adhesive for tissue repair comprising a terpolymer of an xcex1-hydroxycarboxylic acid which exhibits adhesive strength of about 600 to about 150,000 Pa.
This invention also relates to methods for repairing bone or cartilage which comprise applying to the bone or cartilage an implant matrix of this invention.
In the methods of repairing bone or cartilage using a bioerodable implant matrix, this invention further provides the improvement comprising using a tissue-adherent implant matrix of this invention to repair the defect.
In another aspect this invention relates to implantable articles of manufacture for use in the release of a bioactive agent into a physiological environment, said articles comprising a biologically active agent disbursed in an implant matrix of this invention.
The present invention is directed to improvements in matrices for tissue repair comprising biocompatible, bioerodable polymers. In one improvement, the matrix comprises a polymer which has a water solubility of about 0.01 to about 500 mg/mL at about 25xc2x0 C. and an adhesive strength of about 600 to about 150,000 Pa so that the matrix is tissue adherent. One such matrix comprises a polymer that has a glass transition temperature of less than 0xc2x0 C. The improved matrix can further comprise a filler or a bioactive agent, or both. An especially useful attribute of the improved matrices is that the matrix adheres to tissues such as bone or cartilage. In addition, the matrix has a texture like that of dough or putty; thus, it is particularly suitable for being molded to fit into a site needing repair.
In another aspect, this invention provides pressure sensitive adhesives for tissue repair comprising (a) a biocompatible, bioerodable polymer which exhibits adhesive strength of about 600 to about 150,000 Pa, (b) a filler and (c) a bioactive agent. Further, this invention provides pressure sensitive adhesives for tissue repair comprising a terpolymer of an xcex1-hydroxycarboxylic acid which exhibits adhesive strength of about 600 to about 150,000 Pa.
The implant matrices and adhesives of this invention can be applied to the bone-contacting surfaces of prosthetic appliances (as a cement), or they can be inserted into and around bone defects and cavities or cartilage surfaces (as a filler). The matrix or adhesive biodegrades gradually. As it biodegrades, it is replaced by developing bone or cartilage tissue in a manner which permits a natural healing of the tissue. Thus, it provides an effective means for treating or repairing bone or cartilage.
When the matrix or adhesive further comprises a bioactive agent, it serves as a depot device for release of the bioactive agent. Release of the agent occurs as the matrix or adhesive biodegrades after implantation.
Many attempts have been made to develop a repair matrix that could facilitate bone or cartilage repair and also deliver bioactive agents such as growth factors. Such a matrix could be used instead of bone grafts. Thus far, only matrices comprised of natural products such as collagen have shown promise. Collagen, however, is difficult to manufacture and control in order to meet regulatory standards. In addition, surgeons are not satisfied with collagen matrices because they are difficult to form and/or handle.
Other approaches to replace bone grafts have included conventional bioresorbable polymers, ceramics such as tricalcium phosphate (TCP), natural polymers, such as collagen, proteoglycans, starches, hyaluronic acid and modified bone matrix. To date these efforts have only produced delivery matrices which (a) impede healing, (b) provoke negative tissue reactions, (c) cannot be sterilized, (d) are difficult to use or (e) cannot be manufactured to the satisfaction of regulatory bodies.
For example, one approach was to use conventional bioresorbable polymers such as polylactide-co-glycolide (PLG) to administer growth factors. It was very difficult, however, to combine PLG with the growth factor without inactivating the growth factor. Other disadvantages encountered with PLG were that, when it was implanted, it inhibited the bone healing response and occasionally caused aseptic sinus tract and inflammation and destroyed surrounding bone.
Another attempt to develop an effective bone repair matrix involved implanting a bone growth factor absorbed on a ceramic such as TCP. The problem with this approach was that the TCP particles migrated out of the defect area too quickly to deliver the growth factor effectively.
A major problem encountered with previously tried delivery systems is that the bioerodable material could not be mixed with the growth factor prior to the time of surgery. Mixing the delivery matrix with the bioactive material immediately prior to, or during, the surgery process is very awkward and can lead to inconsistent results.
The bioerodable matrices and adhesives of this invention solve several of the problems encountered with previous delivery systems. They are especially useful in the delivery of bioactive proteins such as growth factors because the polymer component dissolves in solvents which are compatible with proteins. Thus, it is possible to formulate the bioactive component in the polymer adhesive matrix in advance, i.e., well before a surgical procedure, under acceptable regulatory conditions, including sterilization of the product without inactivating the bioactive components. Quality control during the preparation of delivery systems using the present adhesive products is, therefore, greatly improved.
Other advantages of the polymer implant matrices and adhesives of this invention are that they are biocompatible and bioerodable in vivo. The term xe2x80x9cbiocompatiblexe2x80x9d means that the polymer is non-toxic, non-mutagenic and, at most, elicits only a minimal to moderate inflammatory reaction. The term xe2x80x9cbioerodablexe2x80x9d means that the polymer either degrades or is resorbed after implantation into products that are used by, or are otherwise eliminated from, the body by existing biochemical pathways.
The present matrices comprise polymers that are bioerodable within a period of from about three hours to about two years. This period can be varied, depending upon the desired application. A preferred period is from about one day to about one month; another preferred period is from about two weeks to about three months. The period for bioerosion is the time after which the polymer will no longer be detectable at the site of implantation, using standard histological techniques.
Thus, an important advantage of the present polymer implant matrices is that a second surgical procedure to remove the matrix is not required because it degrades with time, and its degradation products are absorbed by the body.
One required feature of certain of the adhesive bioerodable polymers useful in the improved matrices of this invention is their water solubility. They are soluble in water at about 0.01 to about 500 mg/mL of water at about 25xc2x0 C. (ambient temperature). Typically, the polymers are soluble in water at about 0.1 to about 500 mg/mL of water. Preferably they are soluble at about 5 to about 400 mg/mL of water.
Some investigators have reported aseptic necrosis, inflammation, or sinus tracts in animals where poly(xcex1-hydroxycarboxylic acid) implants have been used. It is generally thought that these adverse reactions were caused by local acidosis from the degradation of the polymer. Use of more soluble ionomer forms of the polymers avoids the danger of developing local acidosis at implant sites because the polymers dissolve and are diluted or carried away before quantities of acidic degradation products are produced.
This water solubility allows the polymers to be more readily dissolved by serum at the surface of the implant matrix and thereafter distributed into surrounding body fluids where they can be mobilized for hydrolysis at remote sites. This feature is important because hydrolysis of some polymers results in a localized pH gradient which can be adverse to local cell growth. Hydrolysis occurring at the implant site produces an unnatural concentration of hydrolysis products (and increased acidity) at the surface of the matrix. Such acidity can easily interfere with ongoing tissue repair. The water soluble polymers used in the improved matrices of this invention, therefore, preserve conditions that optimize a localized environment for cell viability and growth at the implant surface.
Certain polymers used in the matrices of this invention, the polyesters, have a glass transition temperature (Tg) of less than 0xc2x0 C. When used with a filler, polymers with a Tg of less than 0xc2x0 C. have excellent handling properties.
A required feature of all the polymers for use in the matrices and adhesives of this invention is a threshold level of adhesiveness. Adhesiveness has been found to be important for optimizing implant performance. Adhesiveness is an intrinsic property that is not readily correlated with polymer properties, but can easily be assessed empirically. Adhesiveness is a characteristic that derives from a wide variety of polymer parameters, including polymer type, i.e., the nature of the covalent linkages linking the monomers, molecular weight and intrinsic structure and as well the nature of the surface to which the matrix will be adhered. Skilled practitioners in the art can readily assess polymer adhesive properties using known techniques, such as those illustrated in the examples infra.
The polymers used in the matrices and adhesives of this invention exhibit adhesive properties on different substrates, such as, for example, dry substrates like glass and water-swollen poly(2-hydroxyethyl methacrylate) (xe2x80x9cpHEMAxe2x80x9d) on glass, which simulates wet tissues. Typically, the polymers withstand a maximum stress on a glass substrate of about 1,000 to about 150,000 Pa, preferably about 10,000 to about 40,000 Pa, and most preferably, about 12,000 to about 16,000 Pa. The polymers withstand a maximum stress on a pHEMA substrates of about 600 to about 90,000 Pa, preferably about 2,500 to about 40,000 Pa, and most preferably about 5,500 to about 8,500 Pa. Thus, the range of adhesive strength is from about 600 to about 150,000 Pa.
The polymers are moldable by hand at a temperature of about 60xc2x0 C. or below. Typically, they are moldable at about 4xc2x0 C. to about 60xc2x0 C., preferably at about 15xc2x0 C. to about 50xc2x0 C., and most preferably at about 20xc2x0 C. to about 30xc2x0 C. The degree of moldability at a selected temperature is dependent upon the characteristics of the polymer selected as well its molecular weight. The matrix containing the polymer remains moldable after it has been implanted within the body.
A variety of polymers can be used in the matrices and adhesives of this invention. The polymers must be biocompatible and susceptible to rapid biodegradation in order to be replaced by new tissue. The polymers may be homopolymers, terpolymers, copolymers, blocked copolymers, or blends of polymers. Bioerodable polymers include polyanhydrides, polyorthoesters, polyesters (such as polylactic acid (PL), polyglycolic acid (PG), polyhydroxybutyric acid, polymalic acid, polyglutamic acid and polylactones) and poly(amino) acids.
One type of polymer especially useful in the matrices and adhesives of this invention is a polyester ionomer, more particularly, a non-toxic salt of a bioerodable carboxy-terminated polyester of the general formula ROxcx9cPExcx9cCOOH or HOOCxcx9cPExcx9cCOOH wherein R is hydrogen or C1-C4 alkyl and xcx9cPExcx9cis a divalent residue of a polyester. The polyester can comprise a homopolymer, copolymer, or terpolymer of biocompatible hydroxy acids, for example, lactic acid, glycolic acid, xcex5-hydroxycaproic acid, and xcex3-hydroxyvaleric acid. Alternatively, the polyester can be formed using copolymerization of a polyhydric alcohol and a biocompatible polycarboxylic acid. Most typically such copolymers are formed between dihydric alcohols, for example, propylene glycol for biocompatibility and biocompatible dicarboxylic acids. Representative carboxylic acids for formation of the polyesters useful for preparing these polyester ionomers include a Kreb""s cycle intermediate such as citric, isocitric, cis-akonitic, xcex1-ketoglutaric, succinic, maleic, oxaloacetic and fumaric acid. Many of such carboxylic acids have additional functionalities which can enable further cross-linking of the polymers if desirable.
The polyesters can be further modified, for example, by reaction with a cyclic carboxylic anhydride to convert residual hydroxy functionality to the carboxy-terminated forms useful for preparation of these polyester ionomers.
The carboxy-terminated polyesters used to prepare the polyester ionomers are selected to have a threshold water solubility between about 0.01 and about 500 mg/mL of water, preferably about 0.5 to about 350 mg/mL of water, at ambient temperature. The polyester precursors have a weight average molecular weight of about 400 to about 10,000, more typically about 1,000 to about 5,000. Conversion of these compounds by neutralization with pharmaceutically acceptable bases produces polyester ionomers having enhanced water solubility relative to the carboxy-terminated polyester precursors but retaining other polymer functionality.
The polyester ionomers are prepared from mono-or bis-carboxy-terminated polyesters. Generally, the carboxy-terminated polyester is dissolved in an organic solvent and neutralized by the addition of a stoichiometric amount of a physiologically acceptable base. In one embodiment, the neutralization is carried out with less than a stoichiometric amount of base to produce a composition comprising a carboxy-terminated polyester and its corresponding ionomer, the ratio of those components being dependent on the degree of neutralization. Suitable bases for use in forming the polyester ionomers include hydroxides of Group Ia or Group IIa metals including preferably the hydroxides of lithium, sodium, potassium, magnesium and calcium, as well as physiologically compatible salt-forming amines. Following neutralization of the carboxy-terminated polyester, the resulting ionomer can be isolated using standard isolation techniques the ionomer is typically dried prior to use in fabrication of implant matrices and adhesives.
The carboxy-terminated polyesters can be prepared using art-recognized procedures for polyester synthesis. The carboxy-terminus (or termini) on such compounds can be formed by reaction of hydroxy functional polyesters with, for example, a stoichiometric amount of a cyclic anhydride of a C1-C6 dicarboxylic acid, such as succinic anhydride.
Bis-hydroxy functional polyesters are readily prepared by reaction of a dihydric alcohol initiator, for example, propylene glycol or ethylene glycol, with one or more cyclic hydroxy acid esters, for example lactide, glycolide or caprolactone. Reaction of such bis-hydroxy functional polyesters with cyclic anhydrides produces bis-carboxy functional polyesters that can be used to prepare the ionomers described supra.
The polyester prepolymers used for the preparation of the ionomers can be prepared using art-recognized polyester-forming reaction chemistry, typically using, for example, metal catalysts to promote ester-forming reactions. One problem with the prior art procedures is the difficulty in removing the metal catalyst from the product polyesters. Removal of the catalyst is particularly crucial when the polyesters are intended for use in medical applications.
It has been found that polyesters of hydroxy acids can be prepared in high yields and high purity with good control over structure/functionality by reacting the corresponding cyclic esters with a hydroxy functional initiator at elevated temperatures under substantially anhydrous conditions. Thus, one preferred method for preparing the polyesters consists of reacting an initiator, for example, a mono-hydric or dihydric alcohol, with at least one cyclic hydroxy acid ester under substantially anhydrous conditions at elevated temperatures. The reaction is preferably carried out neat (an absence of solvent) at a temperature of about 100-180xc2x0 C., more preferably about 120-160xc2x0 C. The term xe2x80x9csubstantially anhydrous conditionsxe2x80x9d means that routine efforts are made to exclude water from the reaction mixture and can typically include such steps as pre-drying the reaction vessel with heat and carrying out the reaction under drying conditions.
The structure of the polyester is controlled by selection and stoichiometry of the cyclic hydroxy acid ester reactant(s) and the amount of initiator used with lower relative initiator amounts leading to higher average molecular weight product and higher relative amounts of initiator leading to lower average molecular weight product.
The hydroxy functional initiator can either be a monohydric alcohol, for example a C1-C4 alkanol, or a di-or polyhydric alcohol. Alternatively, the hydroxy functional initiator can be a hydroxy acid, for example glycolic acid. The product hydroxy-terminated polyesters can be converted to a carboxy-terminated polyester that can be used to prepare the polyester ionomers by reaction with a stoichiometric amount of a cyclic anhydride.
The method for preparing polyester polymers for use in preparing the polyester ionomers can be carried out as well in the presence of a cyclic carboxylic acid anhydride to provide directly the corresponding carboxy terminated polyester compound. The reaction is carried out under the same conditions described supra for preparing the polyester. Most typically the reaction is carried out using about equimolar amounts of a monohydricalcohol initiator and the cyclic anhydride. Where the initiator is a dihydric alcohol, the molar ratio of the cyclic anhydride to the initiator is preferably raised to about 2:1.
Preferred polyester ionomers are those made up of lactide, glycolide and caprolactone or valerolactone. Polymers of lactide/glycolide/caprolactone (PLGC) are especially beneficial. PLGC terpolymers having a molecular weight in the range of about 1,000 to 3,000 are especially preferred. Terpolymers wherein the lactide and glycolide each make up about 35-45% of the terpolymer, and the caprolactone or valerolactone make up about 10 to about 30% of the terpolymer are particularly useful.
Selected poly(amino acids)are another type of polymer useful in the matrices and adhesives of this invention. Certain poly(amino acids) exhibit adhesive properties toward connective tissue, such as cartilage and bone. The poly(amino acid) can be: (1) a classic poly(amino acid) of the formula H2Nxe2x80x94Qxe2x80x94COOR2 in which Q is the divalent residue of a polylactide and R2 is H, a metal cation, or ammonium, or (2) a pseudo-poly(amino acid).
The matrix may comprise two or more different poly(amino acids), each of the formula H2Nxe2x80x94Qxe2x80x94COOR2 wherein:
Q is a divalent residue of a polylactide formed from 1 to 3 species of amino acids;
the amino acid components of Q are represented by the formula aX+bY+cZ;
wherein a, b, and c represent the respective mole fractions of the amino acids X, Y, and Z; a=0 to 1, b=0 to 1, and c greater than 0 but less than 1; and a+b+c=1.0;
X is selected from glutamate, asparagine, aspartate, and glutamine;
Y is selected from lysine and arginine; and
Z is selected from cysteine, methionine, serine, threonine, glycine, alanine, valine, leucine or isoleucine.
Alternatively, the matrix may comprise a divalent or multivalent monomer and a poly(amino acid) of the formula H2Nxe2x80x94Qxe2x80x94COOR2 as defined supra wherein the Q polylactide is formed from 1 to 3 species of amino acids.
A wide variety of polypeptides in a wide variety of ratios may be used to form the useful poly(amino acids). The polypeptides are available commercially from Sigma Chemical Company, P.O. Box 14508, St. Louis, Mo. 63178.
Certain amino acid homopolymers, however, are not useful in the matrices. For example, amino acids with aliphatic side chains do not interact well enough with biological surfaces. They may, however, be used as chain extenders or modulators, along with cysteine, methionine, serine, and threonine, in mixed polymers. Amino acids with aromatic side chains exhibit low rates of diffusion in the body and are, therefore, not suitable to be components of selected poly(amino acids). Histidine is also not a suitable component due to its limited interaction with biological surfaces. Histidine may, however, be used to complex with the polyamino acids as a monomer.
Particular divalent or multivalent monomers may be used in combination poly(amino acids) in the matrices. Amino acids with two or more positive charges at physiological pH, such as lysine, arginine, or histidine, form complexes with poly(amino acids) bearing negative charges at physiological pH. Likewise, amino acids with two or more negative charges, such as aspartate or glutamate, can form complexes with poly(amino acids) bearing positive charges.
In the pseudo-poly(amino acids) that can be selected, the dipeptide monomers are covalently bound through other than normal peptide linkages. Pseudo-poly(amino acids) suitable for use are those having the requisite adhesive character. They can be prepared using the chemistry described, for example, in Kohn, J. and Langer, R., Polymerization Reactions Involving the Side Chains of xcex1-L-Amino Acids, J. Amer. Chem. Soc., 109, 917 (1987) and Pulapura, S. and Kohn, J., Biomaterials Based on xe2x80x9cPseudoxe2x80x9d-Poly(Amino Acids): A Study of Tyrosine-Derived Polyiminocarbonates, J. Polymer Preprints, 31, 23 (1990), which are incorporated by reference. The pseudo-poly(amino acids) can be used alone or in combination with a classic poly(amino acid) or with a different pseudo-poly(amino acid).
As discussed supra, the composition of the polymer, as well as the molecular weight and physical properties, can be varied. Those in the art will also appreciate that compounds can be mixed into, or polymerized with, the polymer as required for additional strength or other desirable physical properties, using materials known in the art. For example, TCP or other ceramic-type materials that provide increased viscosity can be added to the composition.
The dissolution rate of polymers such as the PLGC terpolymers can be varied by end group modification. For example, PLGC terpolymers with OH end groups degrade very slowly; PLGC terpolymers wherein the OH end groups have been partially neutralized, e.g., by neutralization of about 40 to 60% of the end groups with sodium hydroxide, degrade at a moderately slow rate; and PLGC terpolymers wherein most the OH end groups have been neutralized, e.g. by sodium hydroxide, degrade within a few days. Exemplary end groups are OH and COONa+, but any ion or functional group that can be placed on the polymers could be used. The amount of end group modification can have a dramatic effect on the dissolution rate.
In addition to end group changes, variations of molecular weight and composition can be selected to prepare suitable compositions. Increases in molecular weight increase the time to dissolution. Also, blending in a high MW polymer will increase the time to dissolution, or blending in a low MW polymer will decrease the time.
In general, when the matrix is used to repair bone defects, the polymer is selected to degrade over a period of three hours to two years. Preferably, the polymer will degrade in about one month, most preferably in about two weeks. The desired degradation time will depend on the nature of the repair site, including the local tissue type, the support function being served by the implanted matrix, and the nature and concentration of the bioactive component, if any, in the implant matrix. Targeted degradation times can be achieved by selection of polymer/filler combinations on an individual basis.
In the matrix, the polymer may be combined with a bioactive agent, one or more fillers, or both. When the matrix contains a filler, it typically contains about 1 to about 90 weight percent filler, preferably about 30 to about 70 weight percent, and most preferably about 35 to about 50 weight percent of filler.
The filler may be particulate, fibrous, organic, inorganic or a mixture of organic and inorganic. Suitable fillers include bone chips, tricalcium phosphate, hydroxylapatite (xe2x80x9cHAxe2x80x9d), small intestine submucosa (xe2x80x9cSISxe2x80x9d as described in U.S. Pat. No. 4,902,508, issued Feb. 20, 1990, and U.S. Pat. No. 4,956,178, issued Sep. 11, 1990), bioglass granules, synthetic polymers, calcium carbonate, calcium sulfate and collagen, or other extracellular matrix compound, or various mixtures thereof.
When the filler is particulate, the average particle size is from about 20 xcexcm to about 2,000 xcexcm, more preferably about 75 to about 700 xcexcm, and most preferably, about 100 xcexcm to about 500 xcexcm.
As discussed supra, the implant matrix may contain a bioactive agent or agents. A bioactive agent is a compound or material that affects the living cells in its surrounding environment, e.g., it acts to enhance the healing process.
Bioactive agents preferred for use in the present invention are growth factors, growth factor binding proteins or cells. Examples of suitable growth factors include: a fibroblast growth factor, a transforming growth factor (e.g., TGF-xcex21), a bone morphogenetic protein, epidermal growth factor, an insulin-like growth factor or a platelet-derived growth factor.
Examples of growth factor binding proteins are insulin-like growth factor binding proteins (IGFBP""s) such as IGFBP""s 3 and 5. Examples of suitable cells include bone marrow cells and mesenchymal stem cells. The bioactive material can also be an osteogenic agent which stimulates or accelerates generation of bone upon implantation into a bone defect site. Examples of osteogenic agents include demineralized bone powder, morselized cancellous bone, aspirated bone marrow, bone forming cells, and other bone sources.
The bioactive agent may also be an antibacterial substance. Examples of useful antibacterial agents include gentamicin and vancomycin.
When a bioactive agent is included in the matrix or adhesive, it is incorporated in amounts of from about 10xe2x88x925% to about 33% by weight of the matrix. Typically, the agent is incorporated at a rate of from about 10xe2x88x922% to about 20% by weight of the matrix. A preferred rate of incorporation is from about 10xe2x88x921% to about 5% by weight.
When the bioactive agent is a growth factor, it is generally incorporated into the matrix or adhesive in amounts from about 10xe2x88x925% to about 1% by weight of the matrix. When cells are the active component, the range is from about 0.5% to about 50% by weight. When using an agent such as demineralized bone, bone marrow and the like, the range is preferably from about 5% to about 95% by weight. For TGF-xcex21 the preferred range is from about 10xe2x88x924% to about 0.05% of TGF-xcex21 by weight of the matrix.
The percent of bioactive agent should be such that it will release from the implanted matrix in vivo in an effective manner, generally over a period of from about a day to about 30 days and longer, depending on the nature and application of the composition.
The release rate of a bioactive agent, such as TGF-xcex21, can be varied by modification of the polymer as discussed supra, e.g., by varying its end groups, molecular weight or composition.
Other agents that may be added to the matrix include: an extract from whole blood, packed red cells, platelets, plasma (fresh or fresh frozen), serum, skin, bone, cartilage, tendon or microorganisms; synthetic proteins, etc. Suitable proteins can be any one of a wide variety of classes of proteins, such as keratins, collagens, albumins, globulins, hormones, enzymes, or the like. The material can be simple peptides, simple proteins, or conjugated proteins, such as glycoproteins, mucoproteins, lipoproteins, heme proteins, nucleoproteins, or the like.
Antioxidants may also be included in the matrix. Antioxidants suitable for use include tocopherol, citric acid, butylated hydroxyanisole, butylated hydroxytoluene, tert-butylhydroquinone, propyl gallate, sodium ascorbate, and other antioxidants which are xe2x80x9cgenerally recognized as safexe2x80x9d by the Food and Drug Administration.
Thus, the implant matrices can be prepared by blending the polymer with one or more bioactive agents and optionally other excipients, for example, additives to optimize retention of biological activity and polymer functionality during sterilization, and then by sterilizing and packaging the implant formulation for surgical use.
Sterilization can be accomplished by radiation with about 1 to about 3 mRad of gamma radiation or electron beam radiation. If the bioactive agent is a protein or peptide, biological activity can be optimized during sterilization by including in the formulation 1) an extraneous protein, for example albumin or gelatin; and 2) a free radical scavenger (antioxidant), for example propyl gallate, 3-tert-butyl-4-hydroxyanisole (BHA) or ascorbic acid, in amounts effective to retard radiation-induced degradation of the biologically active peptide. The sterilization is preferably conducted at low temperature, for example xe2x88x9270xc2x0 C.
When a filler is used in the matrix with a biologically active peptide or protein, it is advantageous to form a mixture of the biologically active compound and an extraneous protein such as albumin or gelatin, and coat the filler with that formulation prior to blending the filler into the polymer.
Preferred matrices for bone repair include the following:
The implant matrices of this invention can be prepared using standard formulation techniques. If the matrix includes a bioactive agent, the polymer can be mixed with the agent or used to encapsulate it, again using known methods such as mixing and compressing and microencapsulation.
This invention also provides an implantable article of manufacture for use in the release of a bioactive agent into a physiological environment comprising a biocompatible tissue-adherent implant matrix of this invention and one or more bioactive agents. Preferred implantable articles are those wherein the bioactive agent is a growth promoting factor.
Although the polymers have been described for use in repairing tissues such as bone and cartilage and in a delivery matrix for a bioactive agent in vivo, these descriptions are illustrative only and are not intended to be limiting in any way. There are many other applications for the bioerodable adhesive polymers of this invention.
For example, the polymers can be used in the treatment of bone tumors. Such treatment typically involves excision of the tumor as well as portions of the surrounding bone, leaving a large cavity in the bone. A graft using autogenous bone (bone harvested from another site in the patient""s body) is the conventional and accepted technique for filling such bony defects. Although use of autogenous bone provides rapid incorporation of new bony ingrowth into a bone cavity, this procedure is associated with a morbidity caused by the required surgical exposure needed to harvest the patient""s bone. Moreover, some patients, particularly osteoporotic individuals, have very limited amounts of bone that are appropriate for use as a graft.
Alternatively, allografts, i.e., bones taken from other individuals, may be used as bone-grafting material. There are certain risks associated with such allografts, however, including the transfer of infections and even unrecognized malignant cells from the harvested patient to the grafted patient as well as the problem of immunologic barriers between all individuals. Furthermore, these processes are complicated and labor-intensive. Thus, the implant matrices of this invention offer a distinct improvement over traditional treatments for bone tumors.
The polymers used in the matrices and adhesives of this invention are typically prepared so that they form a viscous adhesive rather than a conventional solid. When the polymer is mixed with a particulate filler to form the biocompatible matrix or adhesive, the polymer can be used to coat the particles of filler. An example of a suitable particulate filler is a ceramic such as TCP. When the particles are coated with the polymer adhesive, they form a self-adherent dough-like substance that can be conveniently molded to fit surgically into bone defects. When a bioactive agent, such as a protein growth factor, is to be included in the matrix, it can be absorbed onto the particles of the biocompatible solid filler prior to being coated with the polymer adhesive.
This invention also relates to improvements in methods of repairing bone or cartilage using a bioerodable implant matrix, wherein the improvement comprises using a tissue-adherent matrix of this invention to repair the bone or cartilage.
Preferred improvements are those wherein the matrix contains a bioactive agent, particularly those wherein the bioactive agent is a growth-promoting factor.
When using a matrix of this invention to repair bone or cartilage, a surgeon, physician or other caregiver first determines the size of the cavity or void to be filled, or the dimensions of the repair site, and removes the appropriate amount of polymer adhesive matrix from packaging. Typically, the packaging is a barrier package which prevents water vapor from contacting the polymer in the composition; however, it is understood that the packaging may be any one of a wide variety of containers.
Following removal from the packaging, the surgeon then molds the adhesive implant matrix at ambient temperature into dimensions compatible with the repair site. In the case of bone repair, the matrix is molded to the dimensions of the cavity or void to be filled. In the case of connective tissue repair, it is molded to fit the dimensions of the repair site. The adhesive matrix is then applied to the cavity or repair site in a manner which permits it to adhere to the bone or cartilage for a time sufficient to effect its repair. Typically, the surgeon presses the molded matrix against the damaged, and often wet, tissue. Because the matrix has adhesive properties, when it is applied to the surrounding bone or connective tissue with pressure, it will stick and remain in place long enough to effect repair of the bone or tissue.
When the matrix contains a bioactive agent, it is typically implanted in a site in the body where a concentration of the bioactive agent would be beneficial. Thus, for example, in the treatment of an osteoporosis-induced fracture involving a void or bony defect, an implant matrix containing a growth-promoting agent is molded to conform to the bone defect or cavity and is inserted by the surgeon at that location. Similarly, the matrix can be implanted or injected into soft tissue for sustained drug release.
Instrumentation. Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples with respect to polystyrene standards (Polysciences Corporation). The system configuration was described by R. F. Storey and T. P. Hickey, J. Polymer Sci. 31, 1825 (1993). Throughout this specification, unless otherwise stated, the term xe2x80x9cmolecular weightxe2x80x9d refers to weight average molecular weight.
General Procedure
1. Synthesis of acid-terminated polymers
Glassware was dried at 145-155xc2x0 C. for 24 h, fitted with rubber septa, and cooled under a flow of dry nitrogen. Polymerizations were run in 250-mL Erlenmeyer flasks with 24/40 ground glass joints sealed with evacuated glass stoppers wrapped with teflon tape. To a flask (250 mL) containing a magnetic stir bar were added D,L-lactide (18.17 g, 1.26xc3x9710xe2x88x921 mol), glycolide (14.63 g, 1.26 xc3x9710xe2x88x921 mol), xcex5-caprolactone (7.20 g, 6.30xc3x9710xe2x88x922 mol), glycolic acid (1.66 g, 2.18xc3x9710xe2x88x922 mol), succinic anhydride (2.19 g, 2.18xc3x9710xe2x88x922 mol). The flask was purged with nitrogen and heated in a 135xc2x0 C. constant temperature bath for 20 h with continuous stirring. At 65 h of reaction, the temperature was lowered to 110xc2x0 C. The polymerization was allowed to proceed for 146 h and was then quenched in an ice-water bath.
2. Analytical titration procedure (2,000 g/mol sample)
To a 125-mL Erlenmeyer flask was added a (xcx9c2,000 g/mol) polymer sample (0.30-0.40 g). The polymer sample was completely dissolved in THF (50 mL), and water (15 mL) was added to the solution. Phenolphthalein (1 g/100 mL MeOH) (5 drops) was added to the polymer solution, and the flask was placed in an ice bath. The sample was titrated with an aqueous solution of NaOH (0.5047 N) to a light pink end point. An average equivalent weight was calculated from the values of at least three titrations.
3. Bulk polymer titration procedure (2,000 g/mol sample)
To a 1,000-mL Erlenmeyer flask was added a (xcx9c2,000 g/mol) polymer sample (34.32 g), and the polymer was dissolved in THF (450 mL). The average equivalent weight from analytical titration procedure 2, supra, was used to calculate the exact amount of titrant (85.3 mL, 0.5047 N aqueous NaOH) necessary to completely neutralize the polymer sample. This amount was slowly added to the polymer solution as it was stirred in an ice bath.